1. Field of the Invention
This invention relates generally to methods for creating a tissue effect, and more particularly to methods for creating a tissue effect using an electromagnetic energy delivery device and a reverse thermal gradient.
2. Description of Related Art
The human skin is composed of two elements: the epidermis and the underlying dermis. The epidermis with the stratum corneum serves as a biological barrier to the environment. In the basilar layer of the epidermis, pigment-forming cells called melanocytes are present. They are the main determinants of skin color.
The underlying dermis provides the main structural support of the skin. It is composed mainly of an extra-cellular protein called collagen. Collagen is produced by fibroblasts and synthesized as a triple helix with three polypeptide chains that are connected with heat labile and heat stable chemical bonds. When collagen-containing tissue is heated, alterations in the physical properties of this protein matrix occur at a characteristic temperature. The structural transition of collagen contraction occurs at a specific “shrinkage” temperature. The shrinkage and remodeling of the collagen matrix with heat is the basis for the technology. Although the technology can be deployed to effect other changes to the skin, skin appendages (sweat glands, sebaceous glands, hair follicles, etc.), or subcutaneous tissue structures.
Collagen crosslinks are either intramolecular (covalent or hydrogen bond) or intermolecular (covalent or ionic bonds). The thermal cleavage of intramolecular hydrogen crosslinks is a scalar process that is created by the balance between cleavage events and relaxation events (reforming of hydrogen bonds). No external force is required for this process to occur. As a result, intermolecular stress is created by the thermal cleavage of intramolecular hydrogen bonds. Essentially, the contraction of the tertiary structure of the molecule creates the initial intermolecular vector of contraction.
Collagen fibrils in a matrix exhibit a variety of spatial orientations. The matrix is lengthened if the sum of all vectors acts to lengthen the fibril. Contraction of the matrix is facilitated if the sum of all extrinsic vectors acts to shorten the fibril. Thermal disruption of intramolecular hydrogen bonds and mechanical cleavage of intermolecular crosslinks is also affected by relaxation events that restore preexisting configurations. However, a permanent change of molecular length will occur if crosslinks are reformed after lengthening or contraction of the collagen fibril. The continuous application of an external mechanical force will increase the probability of crosslinks forming after lengthening or contraction of the fibril.
Hydrogen bond cleavage is a quantum mechanical event that requires a threshold of energy. The amount of (intramolecular) hydrogen bond cleavage required corresponds to the combined ionic and covalent intermolecular bond strengths within the collagen fibril. Until this threshold is reached, little or no change in the quaternary structure of the collagen fibril will occur. When the intermolecular stress is adequate, cleavage of the ionic and covalent bonds will occur. Typically, the intermolecular cleavage of ionic and covalent bonds will occur with a ratcheting effect from the realignment of polar and nonpolar regions in the lengthened or contracted fibril.
Cleavage of collagen bonds also occurs at lower temperatures but at a lower rate. Low-level thermal cleavage is frequently associated with relaxation phenomena in which bonds are reformed without a net change in molecular length. An external force that mechanically cleaves the fibril will reduce the probability of relaxation phenomena and provides a means to lengthen or contract the collagen matrix at lower temperatures while reducing the potential of surface ablation.
Soft tissue remodeling is a biophysical phenomenon that occurs at cellular and molecular levels. Molecular contraction or partial denaturization of collagen involves the application of an energy source, which destabilizes the longitudinal axis of the molecule by cleaving the heat labile bonds of the triple helix. As a result, stress is created to break the intermolecular bonds of the matrix. This is essentially an immediate extra-cellular process, whereas cellular contraction requires a lag period for the migration and multiplication of fibroblasts into the wound as provided by the wound healing sequence. In higher developed animal species, the wound healing response to injury involves an initial inflammatory process that subsequently leads to the deposition of scar tissue.
The initial inflammatory response consists of the infiltration by white blood cells or leukocytes that dispose of cellular debris. Seventy-two hours later, proliferation of fibroblasts at the injured site occurs. These cells differentiate into contractile myofibroblasts, which are the source of cellular soft tissue contraction. Following cellular contraction, collagen is laid down as a static supporting matrix in the tightened soft tissue structure. The deposition and subsequent remodeling of this nascent scar matrix provides the means to alter the consistency and geometry of soft tissue for aesthetic purposes.
In light of the preceding discussion, there are a number of dermatological procedures that lend themselves to treatments which deliver thermal energy to the skin and underlying tissue to cause a contraction of collagen, and/or initiate a would healing response. Such procedures include skin remodeling/resurfacing, wrinkle removal, and treatment of the sebaceous glands, hair follicles adipose tissue and spider veins.
Currently available technologies that deliver thermal energy to the skin and underlying tissue include Radio Frequency (RF), optical (laser) and other forms of electromagnetic energy as well as ultrasound and direct heating with a hot surface. However, these technologies have a number of technical limitations and clinical issues which limit the effectiveness of the treatment and/or preclude treatment altogether.
These issues include the following: i) achieving a uniform thermal effect across a large area of tissue, ii) controlling the depth of the thermal effect to target selected tissue and prevent unwanted thermal damage to both target and non-target tissue, iii) reducing adverse tissue effects such as burns, redness blistering, iv) replacing the practice of delivery energy/treatment in a patchwork fashion with a more continuous delivery of treatment (e.g. by a sliding or painting motion), v) improving access to difficult-to-reach areas of the skin surface and vi) reducing procedure time and number of patient visits required to complete treatment. As will be discussed herein the current invention provides an apparatus for solving these and other limitations.
One of the key shortcomings of currently available RF technology for treating the skin is the edge effect phenomenon. In general, when RF energy is being applied or delivered to tissue through an electrode which is in contact with that tissue, the current concentrate around the edges of the electrode, sharp edges in particular. This effect is generally known as the edge effect. In the case of a circular disc electrode, the effect manifests as a higher current density around the perimeter of that circular disc and a relatively low current density in the center. For a square-shaped electrode there is typically a high current density around the entire perimeter, and an even higher current density at the corners.
Edge effects cause problems in treating the skin for several reasons. First, they result in a non-uniform thermal effect over the electrode surface. In various treatments of the skin, it is important to have a uniform thermal effect over a relatively large surface area, particularly for dermatological treatments. Large in this case being on the order of several square millimeters or even several square centimeters. In electrosurgical applications for cutting tissue, there typically is a point type applicator designed with the goal of getting a hot spot at that point for cutting or even coagulating tissue. However, this point design is undesirable for creating a reasonably gentle thermal effect over a large surface area. What is needed is an electrode design to deliver uniform thermal energy to skin and underlying tissue without hot spots.
A uniform thermal effect is particularly important when cooling is combined with heating in skin/tissue treatment procedure. As is discussed below, a non-uniform thermal pattern makes cooling of the skin difficult and hence the resulting treatment process as well. When heating the skin with RF energy, the tissue at the electrode surface tends to be warmest with a decrease in temperature moving deeper into the tissue. One approach to overcome this thermal gradient and create a thermal effect at a set distance away from the electrode is to cool the layers of skin that are in contact with the electrode. However, cooling of the skin is made difficult if there is a non-uniform heating pattern.
If the skin is sufficiently cooled such that there are no burns at the corners of a square or rectangular electrode, or at the perimeter of a circular disc electrode, then there will probably be overcooling in the center and there won't be any significant thermal effect (i.e. tissue heating) under the center of the electrode. Contrarily, if the cooling effect is decreased to the point where there is a good thermal effect in the center of the electrode, then there probably will not be sufficient cooling to protect tissue in contact with the edges of the electrode. As a result of these limitations, in the typical application of a standard electrode there is usually an area of non-uniform treatment and/or burns on the skin surface. So uniformity of the heating pattern is very important. It is particularly important in applications treating skin where collagen-containing layers are heated to produce a collagen contraction response for tightening of the skin. For this and related applications, if the collagen contraction and resulting skin tightening effect are nonuniform, then a medically undesirable result may occur.
There is a need for improved methods for creating tissue effects using electromagnetic energy and a reverse thermal gradient. There is a further need for methods that create tissue effects with reverse thermal gradients which induce the formation of collagen. Yet there is a further need for methods that create tissue effects which use RF electrodes and reverse thermal gradients.